Section 133(6) can be issued during the assessment or even otherwise with prior approval of senior authority. Once a query has been raised vide notice u/s 133(6) and answer has been given in response there would no opportunity for the AO to reopen the case to inquire on the same subject. Many a times 133(6) is issued to call for information, details are filed but no formal order is passed. Later the AO may issue notice u/s 148 raising same subject. In such a case the following ruling could be of assistance to the taxpayer.
In  20 ITR-OL 10 (Bom) the reopening of the assessment was held to be based on change of opinion when the entire issue which was the subject matter of the reasons recorded had been raised during the original assessment proceedings, and the response obtained from the assessee to notice u/s 133(6).